在PoseGraph2D::HandleWorkQueue里，RunOptimization();之后

// 子图的裁剪，如果没有裁剪器就不裁剪
   TrimmingHandle trimming_handle(this);
   for (auto& trimmer : trimmers_) {
     trimmer->Trim(&trimming_handle);
   }
// 这里只有纯定位时才执行
// 裁剪器完成状态，删除裁剪器
   trimmers_.erase(
       std::remove_if(trimmers_.begin(), trimmers_.end(),
                      [](std::unique_ptr<PoseGraphTrimmer>& trimmer) {
                        return trimmer->IsFinished();
                      }),
       trimmers_.end());

trimmers_的添加是 PoseGraph2D构造函数 —— PoseGraph2D::AddTrimmer

先看构造函数部分

if (options.has_overlapping_submaps_trimmer_2d()  )
{
    const auto& trimmer_options = options.overlapping_submaps_trimmer_2d();
    AddTrimmer(absl::make_unique<OverlappingSubmapsTrimmer2D>(
        trimmer_options.fresh_submaps_count(),
        trimmer_options.min_covered_area(),
        trimmer_options.min_added_submaps_count() )  );
}

三个参数:

1. fresh_submaps_count = 2 最新的2个submaps
2. min_covered_area = 3.0 最小未被覆盖的面积
3. min_added_submaps_count = 5 代表在5个submaps之后才会进行该项操作，这个参数不实用，主要是上面两个

Trims submaps that have less than min_covered_cells_count cells not overlapped by at least fresh_submaps_count submaps. 也就是说一些子图如果只有min_covered_cells_count个栅格没有被fresh_submaps_count个子图覆盖，那么这些子图被裁剪掉。

void PoseGraph2D::AddTrimmer(std::unique_ptr<PoseGraphTrimmer> trimmer)
{
  // C++11 does not allow us to move a unique_ptr into a lambda.
  PoseGraphTrimmer* const trimmer_ptr = trimmer.release();
  AddWorkItem([this, trimmer_ptr]() LOCKS_EXCLUDED(mutex_) {
    absl::MutexLock locker(&mutex_);
    trimmers_.emplace_back(trimmer_ptr);
    return WorkItem::Result::kDoNotRunOptimization;
  });
}

类PoseGraphTrimmer的派生类有 PureLocalizationTrimmer 和 OverlappingSubmapsTrimmer2D。前者属于纯定位部分，这里不看。后者其实主要就实现了一个Trim函数。

回头看trimmers_.erase这一句，发现OverlappingSubmapsTrimmer2D::IsFinished()永远返回 false，所以这一句只有纯定位时才会执行，建图不执行。